1. Technical Field
The present invention relates to a continuously variable transmission with an external, three-dimensional cam and at least four active follower assemblies.
2. Background Information
For a given transmission size, it has been found in the past that contact stress is a limiting factor for the torque capacity of a cam based continuously variable transmission. While other highly stressed components can be resized in order to somewhat accommodate high loads, such as sprag clutches, the input shaft, and the planetary gear system, it has been found herein that the cam and the roller system have a greater effect on overall transmission size. This is because these are the most highly stressed components in the system. In fact, the cam is the largest single component in the continuously variable transmission. It has been found that increasing cam size in order to accommodate larger stresses is to increase the nominal size of the continuously variable transmission as well.
It has been found herein that content stress can be significantly reduced by doubling the number of follower rollers under load at any given time, and by modifying the overall topology of the mechanism to incorporate an inverted and external can surface that surrounds the rest of the mechanism. A unique cable differential, which preferably splits the input torque evenly between two followers in the continuously variable transmission of the present invention, allows for two active followers and therefore loaded follower rollers. Inverting the cam in the present transmission provides a larger radius of curvature, and a more complementary surface for the follower roller to follow.
The spherical roller of the continuously variable transmission of the present invention is on the inner rather than the outer surface of the cam. In addition to the cam being larger as compared to an internal cam, contact stress improvements result from a now negative radius of curvature since the spherical roller is now on the inner surface of the external cam. It has been found herein that contact area between the roller and cam is increased, with less deformation of the roller.
The use of an external cam offers several other improvements in terms of packaging. With an internal cam, there was a large region of space inside the cam which could not be utilized. With the external cam, this space is moved to the outside of the continuously variable transmission, freeing up a large amount of space on the inside. Being as such, the shifting mechanism can be moved to the inside, and the size of the more highly stressed components can be increased while the overall transmission size remains the same.
Although this continuously variable transmission has a uniform output velocity given a constant input, a significant problem with the early prototypes was the non-uniform torque output. This was the result of the heavy follower return springs storing and releasing energy as they held the followers to the cam. Because of the initial internal cam design, large springs were needed to hold the rather massive followers to the cam at high velocities. In addition, the higher follower acceleration required with the dual active followers exacerbated the problem. Indeed, it was experimentally determined that even heavier springs were needed then were built into the design. As such, the external cam will reduce or eliminate the need for large springs because the inertia of the followers will naturally force them onto the cam surface. Therefore lighter springs can be used which are required to only maintain tension in the cables of the cable differential. The torque fluctuations can then be almost entirely eliminated in future prototypes.